Material for neutron shielding and for maintaining sub-criticality based on vinylester resin

ABSTRACT

This invention relates to a composite material for neutron shielding and maintenance of sub-criticality comprising a matrix based on vinylester resin and an inorganic filler capable of slowing and absorbing neutrons. The vinylester resin may be an epoxymethacrylate resin and the inorganic filler may contain a zinc borate and an alumina hydrate or magnesium hydroxide.

TECHNICAL DOMAIN

The purpose of this invention is a material for neutron shielding and maintenance of sub-criticality. This type of material is useful in nuclear energy to protect operators from neutron radiation emitted by radioactive products and to prevent runaway of the neutron formation chain reaction, more particularly when these products contain fissile materials.

In particular, they can be used as neutron shielding in transport packagings and/or for the storage of radioactive products, for example nuclear fuel assemblies.

For neutron shielding, neutrons have to be slowed down and therefore materials containing large quantities of hydrogen have to be used, including the addition of a boron compound to capture neutrons.

To maintain sub-criticality, it is necessary to have a high content of neutron absorber such as boron to prevent runaway of the neutron formation chain reaction.

Moreover, these materials must be self-extinguishing.

STATE OF THE PRIOR ART

Neutron shielding materials obtained from a mix of a high-density inorganic material and a thermosetting resin have been described in EP-A-0 628 968 [1]. In this document, the thermosetting resin may be an unsaturated polyester resin and the inorganic fillers may be heavy metals or compounds of heavy metals.

Document GB-A-1 049 890 [2] describes moulded articles or coatings absorbing neutrons containing at least 0.3% by weight of boron obtained from a co-polymerisable mix of an unsaturated polyester and an unsaturated monomer, in which either the acid component of the polyester is derived partly from boric acid, or the polymerisable monomer is partly a boric acid ester.

Document JP-A-55 119099 [3] describes materials providing protection against neutrons also based on unsaturated polyester resin. This type of material has a hydrogen atoms density equal to 6.1×10²² atoms of hydrogen per cm³, but it does not contain any neutron absorber. Thus, it cannot keep a nuclear fuel transport packaging sub-critical.

These materials based on unsaturated polyester resin have the disadvantage that they have only a mediocre resistance to thermal aging.

PRESENTATION OF THE INVENTION

The purpose of this invention is specifically a material for neutron shielding and maintenance of sub-criticality that has better resistance to corrosion than materials based on unsaturated polyester.

According to the invention, the composite material for neutron shielding and maintenance of sub-criticality comprises a matrix based on vinylester resin and an inorganic filler capable of slowing and absorbing neutrons.

According to the invention, the vinylester resin may be of different types. In general, the resins used are obtained by the addition of a carboxylic acid onto an epoxy resin.

The epoxy resins used have one of two possible types of macromolecular pattern:

-   -   bisphenol A, and     -   novolacquer.

In particular, the carboxylic acid may be acrylic acid or methacrylic acid. Preferably, methacrylic acid is used.

Thus, the vinylester resin is preferably chosen from the group composed of epoxyacrylate resins, epoxymethacrylate resins, bisphenol A type resins, novolacquer type resins and halogenated resins based on bisphenol A.

The epoxyacrylate and epoxymethacrylate bisphenol A type resins may comply with the following formula:

-   -   in which R represents H or CH₃.

The novolacquer type vinylester resins may comply with the following formula:

-   -   in which R represents H or CH₃.

Halogenated vinylester resins based on bisphenol A may also be used according to the invention, for example complying with the following formula:

-   -   in which R is as defined above.

Non-epoxy vinylester resins may also be used in the invention, obtained from isophthalic polyester and urethane, for example complying with the following formula:

-   -   in which R is as defined above and U represents a urethane         group.

Due to the choice of these vinylester resins, the composite material according to the invention has the following advantages.

The atomic concentration of hydrogen in vinylester resins is greater than the atomic concentration of unsaturated polyesters, therefore neutron slowing is better.

These resins have excellent thermal stability and a very good resistance to corrosion, which is advantageous for materials used for neutron shielding and maintenance of sub-criticality, for which usage temperatures are often high.

The material is easy to make since the vinylester resin may be poured directly into the mould that will form the transport or storage packaging for radioactive products.

The loss of mass of shielding materials made of these vinylester resins is low at high temperature.

In the material according to the invention, the vinylester resins have been transformed into a thermosetting material by reaction with a copolymerisable monomer such as styrene and styrene derivatives such as methylstyrene and divinylbenzene, vinyltoluene, methyl methacrylate and allylic derivatives such as diallyl phthalate.

According to the invention, the material also comprises an inorganic filler capable of slowing down and absorbing neutrons, for example metals, metal compounds, boron, boron compounds.

According to the invention, this inorganic filler may in particular comprise at least one inorganic compound of boron and at least one hydrogenated inorganic compound.

Boron compounds that could be used belong to the group comprising boric acid H₃BO₃, colemanite Ca₂O₁₄B₆H₁₀, zinc borates Zn₂O_(14,5)H₇B₆, Zn₄O₈B₂H₂ and Zn₂O₁₁B₆, boron carbide B₄C, boron nitride BN and boron oxide B₂O₃.

Preferably, the composite material according to the invention comprises at least one boron compound chosen from among zinc borate Zn₂O_(14,5)H₇B₆ and boron carbide B₄C.

The hydrogenated inorganic compounds that could be used belong preferably to the group of alumina hydrates and magnesium hydroxide.

The material according to the invention may also include polyvinyl acetate, to make the material non-shrinking.

This material may also comprise a hydrogenated organic filler such as melamine, to improve its self-extinguishing properties.

According to the invention, it is preferable to choose the inorganic compound of boron and the inorganic hydrogenated compound and their quantities so as to obtain a boron concentration in the material equal to 8×10²⁰ to 15×10²¹ of boron atoms per cm³ and a hydrogen concentration of 4×10²² to 6×10²² atoms per cm³.

In the material according to the invention, the quantities of the different constituents are also chosen to obtain density, self-extinguishing and thermal conductivity characteristics suitable for use in a transport and/or storage packaging for radioactive materials.

In particular, it is necessary to have good resistance to aging at a relatively high temperature, since products put in the packaging may reach a temperature of 170° C.

The material also needs to be fire resistant, which means that it should be self-extinguishing, in other words the fire goes out when the flame is removed; and therefore it does not feed the fire.

According to the invention, this self-extinguishing property is conferred particularly by the presence of hydrogenated and/or borated inorganic compounds, for example alumina hydrate or zinc borate.

Similarly, the material should have a low thermal conductivity, but sufficiently high to evacuate heat from transported elements such as irradiated fuel elements.

Finally, as will be seen later, since this material is obtained by pouring a mix of different constituents and a vinyl thinner, it is important that the quantities of the different constituents should be such that the mix has the property that it can be poured. In general, the viscosity of the mix must not exceed 300 Poises.

As an example of a material composition according to the invention, consider the material containing 25 to 40% by weight of thermosetting vinylester resin, in other words including the vinyl thinner, for example styrene.

Preferably, according to the invention, the density of the material is equal to or greater than 1.6, for example 1.65 to 1.9.

Preferably, the materials according to the invention can resist a minimum usage temperature of 160° C.

The material according to the invention may be prepared by setting a mix of constituents in the vinylester resin in solution in a vinyl thinner.

Thus, another purpose of the invention is a process for preparation of the composite material described above, which includes the following steps:

-   -   prepare a mix of vinylester resin in solution in a vinyl thinner         with the inorganic filler,     -   add a catalyst and a setting accelerator to the mix,     -   degas the mix under a vacuum,     -   pour the resulting mix in a mould, and     -   allow it to set in the mould.

The vinyl thinner may for example be styrene, vinyltoluene, divinylbenzene, methylstyrene, methyl acrylate, methyl methacrylate or an allylic derivative such as diallyl phthalate. Preferably, styrene will be used which can both dissolve the vinylester resin and enable setting by copolymerisation.

The catalysts and setting accelerators used are chosen from among compounds normally used for setting of vinylester resins.

In particular, catalysts may be organic peroxides, for example:

-   -   peroxides derived from cetones, such as methylethylcetone         peroxide, acetylacetone peroxide, methylisobutylcetone peroxide,         cyclohexanone peroxide and cumene hydroperoxide;     -   diacyl peroxides, for example benzoyl peroxide, possibly         combined with aromatic tertiary amines such as dimethylaniline,         diethylaniline and dimethylparatoluidine; and     -   dialkyl peroxides such as dicumyl peroxide and ditertiobutyl         peroxide.

The most frequently used accelerators are divalent cobalt salts such as cobalt napththenate or octoate, and aromatic tertiary amines such as dimethylaniline, dimethylparatoluidine and diethylaniline.

One or more additives such as cross-linking inhibitors, surfactants and non-shrinking agents can also be added to the mix.

Examples of inhibitors that could be used include acetylacetone and tertiobutylcatechol.

The method according to the invention is implemented as follows:

The vinylester resin (prepolymer+vinyl thinner) is mixed at ambient temperature with the accelerator(s) and different inorganic fillers, for example hydrogenated and borated fillers. The percentage of fillers may vary from 60 to 75%. These fillers may also provide fire reaction properties. The assembly is mixed so as to obtain a perfectly homogenous mix. The catalyst is added to the mix last. The homogenous mix is then degassed under a vacuum (less than 0.01 MPa). The viscosity of the mix must not exceed 300 Poises (the mix must be pourable).

After degassing, the mix is poured in the required mould in which it is cross-linked to form an insoluble thermosetting material. The mechanism of the reaction is radicalar and the reaction is highly exothermal. The setting time may vary depending on pouring conditions (temperature, catalyst content, accelerator and inhibitor contents). Thus, the gel time may be adjusted by varying the percentages of catalyst and accelerators. The gel time varies from 20 minutes to 2 hours.

According to the invention, the mould used for setting of the resin may be formed directly by the transport and/or storage packaging for radioactive products. For example, the packaging may comprise peripheral recesses in which the mix is poured.

Another purpose of the invention is a transport and/or storage packaging for radioactive products comprising a shield formed from the composite material described above.

Other characteristics and advantages of the invention will become clearer after reading the following description of exemplary embodiments obviously given for illustrative purposes and that are in no way limitative, with reference to the appended drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows mass losses (in %) at 160 and 170° C. of two materials according to the invention as a function of time (in days).

DETAILED PRESENTATION OF EMBODIMENTS

The following examples illustrate the production of composite materials for neutron shielding and maintenance of sub-criticality, containing zinc borate and alumina hydrate or magnesium hydroxide using the resin marketed by Dow Chemical under the trade name Derakane Momentum 470-300 as the vinylester resin.

EXAMPLE 1

A polymerisable mix is prepared from Derakane Momentum 470-300 vinylester resin, styrene, zinc borate Zn₂O_(14,5)H₇B₆ and magnesium hydroxide using the proportions given in table 1 in the appendix.

The following constituents are added to the mix:

-   -   1% by weight, relative to the mass of resin+styrene, of the         55028 accelerator marketed by Akzo, and     -   2% by weight relative to the mass of resin+styrene, of the         Butanox M50 catalyst (methylethyl cetone peroxide) marketed by         Akzo.

The next step is vacuum degassing of the mix for 3 minutes followed by pouring the mix into a mould composed of a compartment of a nuclear fuel transport or storage packaging.

The gel time is 22 minutes at 20° C.

The result is a composite material with the following properties:

-   -   density: 1.697     -   hydrogen content: 4.72% by weight, namely 4.78×10²² atoms/cm³′     -   boron content: 0.97% by weight, namely 9.17×10²⁰ atoms/cm³.

The material obtained has satisfactory thermal properties.

The thermal coefficient of expansion α measured by TMA 40 (METTLER) with a temperature rise of 1⁰° C./minute gives the following for the material:

-   -   α: 35×10⁻⁶ K⁻¹ between 20 and 140° C., and     -   α: 97×10⁻⁶ K⁻¹ above 140° C.

The specific heat Cp is measured by differential enthalpic analysis (DSC 30, METTLER) with a temperature rise rate of 10° C./min, for a temperature range varying from 30 to 200° C.

The values of Cp are within the range 1.19 J.g⁻¹.° C.⁻¹ and 1.89 J.g⁻¹° C.⁻¹ for temperatures between 40° C. and 180° C.

Thermal conductivity measurements are also made for temperatures varying from 25° C. to 180° C. Values are included within the range 0.75 and 0.91 W.m⁻¹K⁻¹.

The mechanical properties of the material are also determined by carrying out compression tests at 23° C. on 10 mm diameter and 20 mm high test pieces, using an Adamel Lhomargy DY26 dynamometer and a test speed of 1 mm/min. The results obtained are as follows:

-   -   compression modulus: 4166±100 MPa,     -   ultimate stress: 155.3±0.8 MPa,     -   compression at failure: 7±0.2%.

Considering the high hydrogen content of the material in example 1, it is particularly suitable for a radiation shielding application.

EXAMPLE 2

The same operating method is used as in example 1, using the constituents and proportions given in table 1.

The mix also includes:

-   -   0.9% by weight relative to the mass of resin, of the accelerator         NL 49P marketed by Akzo, and     -   1.5% by weight relative to the mass of resin, of the Butanox M50         catalyst marketed by Akzo.

Setting takes place at ambient temperature and after 25 minutes, a material with the following characteristics is obtained:

-   -   density: 1.79     -   hydrogen content: 4.80% by weight, namely 5.14×10²² at/cm³,     -   boron content: 0.89% by weight, namely 8.92×10²⁰ at/cm3.

The material obtained has satisfactory thermal properties.

The thermal coefficient of expansion a measured by DSC (METTLER) with a temperature rise of 10° C./min gives the following for the material:

-   -   α: 37×10⁻⁶ K⁻¹ between 20 and 130° C., and     -   α: 109×10⁻⁶ K⁻¹ above 130° C.

The specific heat Cp is measured by differential enthalpic analysis (DSC30, METTLER) with a temperature rise rate of 10° C./min for a temperature range varying from 40° C. to 180° C. Values of Cp are within the range 1.07 and 1.65 J.g⁻¹.° C.⁻¹.

Thermal conductivity measurements are also made for temperatures varying from 20° C. to 170° C. Within this temperature range, the value of the thermal conductivity of the resin is close to 0.8 W/m.K.

The mechanical properties of the material are also determined by carrying out compression tests at 23° C. The compression modulus of the material can thus be found, and is equal to 4299±276 MPa.

Given the hydrogen content, the material in example 2 is particularly suitable for a radiation shielding application.

Thermal aging tests of the material in examples 1 and 2 are also carried out at 160° C., and on the material in example 1 at 170° C.

Aging tests over 6 months consist in putting samples of the material with dimensions 35×25×95 mm into a drying oven at 160° C. and 170° C. and monitoring the mass loss of these samples with time. Variation curves showing the loss of mass of materials (in %) as a function of time (in days) are shown in FIG. 1.

Tests were also carried out on the fire reaction of materials in examples 1 and 2.

Each half-hour fire test at 800° C. was carried out on two 240 mm diameter and 60 mm high blocks of materials in examples 1 and 2. The flame was in direct contact with the material for the first blocks, whereas the second blocks were protected by a 1 mm thick steel plate.

In both cases, and for both materials, self-extinguishing occurs immediately after the torch is removed.

EXAMPLE 3

The same operating method is used as in example 1 to prepare a material for neutron shielding and maintenance of sub-criticality, from the following mix: Derakane Momentum 470- 32% by weight 300 vinylester resin zinc borate 13% by weight boron carbide B₄C 15% by weight alumina hydrate 40% by weight

The mix also comprises:

-   -   0.9% by weight relative to the mass of resin, of the NL49P         accelerator, and     -   1.5% by weight relative to the mass of resin, of the Butanox M50         catalyst.

Setting takes place at ambient temperature; a material with the following characteristics is obtained after 25 minutes:

-   -   density: 1.8     -   hydrogen content: 4.03% by weight, namely 4.34×10²² at/cm³, and     -   boron content: 13.68% by weight, namely 1.37×10²² at/cm³.

Considering its high boron content, the material in example 3 has excellent efficiency in maintaining sub-criticality.

Thus, the material according to the invention has very attractive properties for neutron shielding and maintenance of sub-criticality for the transport of nuclear fuel assemblies.

References Mentioned

-   -   [1] EP-A-0 628 968     -   [2] GB-A-1 049 890

[3] JP-A-55 119099 TABLE 1 Example 1 Example 2 Example 3 (% by (% by (% by Constituents weight) weight) weight) Derakane Momentum 32 32 32 470-300 vinylester resin Added styrene 5 Zinc borate 6.5 6 13 Zn₂O_(14,5)B₆H₇ Boron carbide B₄C 15 Magnesium hydroxide 56.5 Alumina hydrate 62 40 Gel time 22 min 25 min 25 min 

1. Composite material for neutron shielding and maintenance of sub-criticality comprising a matrix based on vinylester resin and an inorganic filler capable of slowing and absorbing neutrons.
 2. Material according to claim 1, in which the vinylester resin is chosen from the group composed of epoxyacrylate resins, epoxymethacrylate resins, bisphenol A type resins, novolac type resins and halogenated resins based on bisphenol A, and resins obtained from isophtalic polyester and urethane.
 3. Material according to claim 2, in which the vinylester resin is an epoxymethacrylate bisphenol A type resin complying with the following formula:

in which R represents H or CH₃.
 4. Material according to claim 2, in which the vinylester resin is a novolac resin of formula:

in which R represents H or CH₃.
 5. Material according to claim 1, in which the inorganic filler comprises at least one inorganic compound of boron and at least one hydrogenated inorganic compound.
 6. Material according to claim 5, in which the inorganic compound of boron is chosen from the group consisting of boric acid H₃BO₃, zinc borates Zn₂O_(14,5)H₇B₆, Zn₄O₈B₂H₂ and Zn₂O₁₁B₆, colemanite Ca₂O₁₄B₆H₁₀, boron carbide B₄C, boron nitride BN and boron oxide B₂O₃.
 7. Material according to claim 5, comprising at least one boron compound chosen among the group consisting of zinc borate Zn₂O_(14,5)H₇B₆, and born carbide B₄C.
 8. Material according to claim 5, in which the hydrogenated inorganic compound is chosen from the group consisting of alumina hydrates and magnesium hydroxide.
 9. Material according to claim 5, in which the quantities of inorganic hydrogenated compound and inorganic compound of boron are such that the boron concentration in the material is equal to 8×10²⁰ to 15×10²¹ of boron atoms per cm³ and that the hydrogen concentration is 4×10²² to 6×10²² atoms per cm³.
 10. Material according to claim 1, comprising 25 to 40% by weight of vinylester resin.
 11. Material according to claim 1, with a density equal to or greater than 1.6, preferably 1.65 to 1.9.
 12. Material according to claim 1, which can resist a minimum usage temperature of 160° C.
 13. Process for preparation of a composite material according to claim 1, including the following steps: prepare a mix of vinylester resin in solution in a vinyl thinner with the inorganic filler, add a catalyst and an accelerator for hardening to the mix, degas the mix under a vacuum, pour the resulting mix in a mould, and allow it to set in the mould.
 14. Process according to claim 13, in which the vinyl thinner is styrene.
 15. Process according to claim 13, in which the mould is a transport and/or storage packaging for radioactive products.
 16. Transport and/or storage packaging for radioactive products comprising a shield formed from a composite material according to claim
 1. 